Integrated optic modulator with reduced capacitance electrode configuration

ABSTRACT

An optic modulator implemented on a substrate that is conductive to light and which includes a pair of light waveguides formed on the substrate is described. The waveguides extend substantially coextensively with one another and are separated from one another by a transverse distance that gradually varies along the courses of the waveguides. The waveguides are arranged to pass between an outer electrode assembly and a center electrode assembly. The outer electrode assembly substantially surrounds the center electrode assembly in such a manner as to create an electric field between the center electrode assembly and the outer electrode assembly when a voltage is applied across the outer and center electrode assemblies. In order to minimize the effective capacitance resulting from the interaction of the outer electrode assembly and the center electrode assembly, the center electrode assembly is implemented using a plurality of individual electrode segments arranged to maximize the electric field strength generated in the area though which the waveguides pass while minimizing the overall effective capacitance. By using such a center electrode configuration, the capacitance is decreased and thus the overall bandwidth of the modulator of the present invention is increased as compared to modulators using a single solid center electrode.

FIELD OF THE INVENTION

The present invention is directed to modulators, and more particularly,to integrated electro-optical modulators with sloped waveguides.

BACKGROUND OF THE INVENTION

Integrated electro-optical modulators are finding increasing use inoptical-fiber communication systems. Such systems are being developed asan alternative to conventional electromagnetic transmission lines.Present optical-fiber communication systems may include, for example, ahigh power, low noise laser source in conjunction with a wide bandwidthexternal modulator.

There are already known various constructions of optic modulators. Amongthe known constructions are modulators which utilize interferencebetween coherent light beams or portions originating at the same lightsource. Such modulators selectively phase-shift the light beams relativeto one another, prior to being combined, in such a manner as to imposethe desired intensity modulation onto the combined light beam.

In this context, it is to be mentioned that integrated optic intensitymodulators that employ optical materials exhibiting the electro-opticeffect, especially lithium niobate or lithium tantalate, offer manyadvantages and, consequently, are becoming increasingly important andpopular in high-performance analog fiber optic links. Applications forsuch modulators include, but are not limited to, cable television,antenna remoting, and phased array radar.

A common problem in such systems is that the dynamic range thereof islimited by intermodulation distortion, and by harmonic distortion due tomodulator nonlinearities.

A first known modulator design, generally indicated by the referencenumeral 100, is illustrated in FIG. 1. FIG. 1 illustrates what is knownas a traveling wave configuration modulator. As illustrated the knowntraveling wave modulator comprises an incoming waveguide segment orchannel 110, a Y input waveguide coupler 112, upper and lower waveguidesegments 114, 116, a Y waveguide output coupler 118 and an outputwaveguide segment 119. Upper and lower outer electrodes 105, 106 arelocated parallel and in close proximity to the upper and lower waveguidesegments 114, 116, respectively. A RF input 102 is coupled to a centerelectrode 103 which is positioned between the upper and lower waveguidesegments 114, 116, respectively. The center electrode 103 is coupled toa terminating impedance 104, e.g., a 50 ohm resistor. The terminatingimpedance 104 is, in turn, coupled to ground.

In the traveling wave configuration, the modulating electronic signaltravels collinearly with the light wave signal which enters through theinput waveguide segment 110 and exits via the output waveguide segment119. In such a design, only one well-defined input is possible to obtainbest frequency response. The frequency response of the traveling waveconfiguration is limited by the characteristic impedance of the coplanartransmission line made up of the electrode structure and the differencein time delay between the microwave signal and the light wave signal asthey propagate along the length of the electrode structure. In order toobtain a good match to the termination impedance and also to obtain asmall differential delay, a dielectric buffer layer between theelectrode and the substrate as well as very thick metallization areusually required. Long electrode structures are sometimes used toprovide high modulation sensitivity. In modulators with such structures,depending on the properties of the buffer layer, thermal heating of thesubstrate can cause instability problems.

While the traveling waveguide modulator configuration offers someadvantages over other designs, in terms of the bandwidths that can beachieved, it also has several disadvantages. In practice, a modulatorimplemented using a traveling wave configuration is subject to designconstraints which make it difficult and awkward to implement. That is,it can be difficult to design a coplanar microwave transmission linewith a characteristic RF impedance approaching 50 ohms. Accordingly,there is a need for a optical modulator that can support relativelylarge bandwidths, e.g., to 1 GHz and beyond, without the use of atraveling wave configuration.

Referring now to FIG. 2, there is illustrated another known opticalmodulator, i.e., a Mach-Zehnder intensity modulator generally indicatedby the reference numeral 200. The optical modulator 200 comprises aninput waveguide segment 210, an input Y-branch waveguide coupler 212,upper and lower waveguide segments 214, 216, an output Y-branchwaveguide coupler 218 and an output waveguide segment 219. Asillustrated a single center feed electrode 207, located between thewaveguide segments 214, 216 is used as a driving electrode. A groundplane 209 surrounds the center electrode 207. Upper waveguide segment214 is positioned between the center electrode 207 and an upper portionof the ground plane 209, while the lower waveguide segment 216 ispositioned between the center electrode 207 and a lower portion of theground plane 209.

As illustrated in FIG. 2, an RF input 202 is coupled to the centerelectrode 207 and to a terminating impedance 204. The terminatingimpedance 204, in turn, is coupled to ground.

The electrode arrangement illustrated in FIG. 2, illustrates a commonapproach to driving the electrode of an optical guided wave modulator.This approach involves the use of a single center electrode 207surrounded by the ground place 209 with the electrode 207 being centerfeed to minimize inductance and resistance. The electrode 207 exhibitsessentially capacitive behavior over low RF drive frequencies, modifiedby interconnect inductance at higher frequencies. The internalresistance of an electronic RF drive signal source, combined with thecapacitance of the electrode 207, defines an RC network which limits thefrequency response of this type of modulator.

The lumped element modulator configuration illustrated in FIG. 2, has atypical cut off frequency of:

    f.sub.c =1/(RC)

where C is the capacitance of the electrode structure and R is theresistance of the drive source, e.g., the resistance 204.

The smaller C is, the higher the upper frequency can be. A small valueof C requires a short electrode interaction length which reduces thesensitivity of the modulator. Also, stray capacitance and inductance ofconnecting bondwires will limit the practical high frequencyperformance.

With the increasing demands for optic modulators with ever largerbandwidths, there is a need for new optic modulators with smallercapacitance values that are capable of supporting the higher bandwidthsrequired.

The optical materials constituting the waveguides and the remainder ofthe substrate and/or the electrodes are capable of not only conductingbut also producing acoustic waves including those which are generated asa result of the application of the different electric fields to theparallel electrodes 209. Such acoustic action and interaction results inripples or fine structure that adversely effect the frequency responseof the modulator 200.

Many attempts have been made in the art to improve the frequencycharacteristics and linearity of lumped electrode type modulators.

U.S. Pat. No. 5,193,128 to Farina, describes the use of waveguides whichare separated from one another by a traverse distance that graduallyvaries along the course of the waveguides in an optical modulator whichuses a solid center electrode. The use of such sloped waveguides reducesand/or spreads the noise distortion in the optical output signal toaffect various light frequencies within the frequency range of interestin a much more uniform manner than when two parallel waveguides areused. This has the effect of reducing if not eliminating the noisedistortion of the optical output signal that is attributable to theeffect of the acoustic waves generated during the operation of themodulator to provide a flatter frequency response than would be possiblewithout the use of sloped waveguides.

Referring now to FIG. 3, there is illustrated a known lumped electrodetype optical modulator 300 with sloped waveguides which reduce theacoustic effects and thus the amount of ripple in the light outputsignal. The optical modulator 300 comprises an input waveguide segment310, an input Y-branch waveguide coupler 312, upper and lowercontinuously sloping waveguide segments 314, 316, respectively, anoutput Y-branch waveguide coupler 318 and an output waveguide segment319. As illustrated, a single solid center electrode 307 is positionedbetween the waveguide segments 314, 316. The solid center electrode 307is used as a driving electrode. A ground plane 309 surrounds the centerelectrode 307. Upper waveguide segment 314 is positioned between thecenter electrode 307 and an upper portion of the ground plane 309, whilethe lower waveguide segment 316 is positioned between the centerelectrode 307 and a lower portion of the ground plane 309.

As illustrated in FIG. 3, an RF input 302 is coupled to the centerelectrode 307 and to a terminating impedance 304. The terminatingimpedance 304, in turn, is coupled to ground.

While the above described known approaches to implementing opticalwaveguide modulators have proved successful to some extent, given modernrequirements for ever larger bandwidths, there is a need for improvedoptical waveguide modulators which can support larger bandwidths than ispossible using the known lumped electrode type modulators. Furthermore,it is desirable that the optical waveguide modulators have a generallyflat frequency response and a linearized output. It is also desirablethat the modulators be capable of being implemented without the use of atraveling wave configuration.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to modulators, and more particularly,to integrated electro-optical modulators with sloped waveguides.

In accordance with one exemplary embodiment, an optic modulator includesa pair of light waveguides formed on a substrate The waveguides extendsubstantially coextensively with one another and are separated from oneanother by a transverse distance that gradually varies along the coursesof the waveguides. The waveguides are arranged to pass between an outerelectrode assembly and a center electrode assembly. The outer electrodeassembly substantially surrounds the center electrode assembly in such amanner as to create an electric field between the center electrodeassembly and the outer electrode assembly when a voltage is appliedacross the outer and center electrode assemblies. In order to minimizethe effective capacitance resulting from the interaction of the outerelectrode assembly and the center electrode assembly, the centerelectrode assembly is implemented using a plurality of individualelectrode segments arranged to maximize the electric field strengthgenerated in the area though which the waveguides pass while minimizingthe overall effective capacitance. By using such a center electrodeconfiguration the surface area of the center electrode is reduced ascompared with the surface area of a solid electrode. Because of thedecreased center electrode surface area the capacitance is decreased andthus the overall bandwidth of the modulator of the present invention isincreased as compared to modulators using a single solid centerelectrode. It is expected that a decrease in capacitance of between 20and 40 % may be achieved using the center electrode assembly of thepresent invention as opposed to a solid center electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial diagram illustrating a known optical modulatorimplemented in a traveling wave configuration.

FIG. 2 is a pictorial diagram illustrating a known optical modulatorimplemented in a lumped element configuration.

FIG. 3 is a pictorial diagram illustrating a known optical modulatorincluding sloped waveguides and a solid center electrode implemented ina lumped element configuration.

FIG. 4 is a pictorial diagram illustrating an optical modulatorincluding a multiplate center electrode implemented in accordance withone embodiment of the present invention.

FIGS. 5 through 7 are pictorial diagrams illustrating various opticalmodulators including sloped optical waveguides and multisegmentedelectrodes implemented in accordance with various embodiments of thepresent invention.

FIGS. 8 through 10 are pictorial diagrams illustrating variouslinearized optical modulators which include a plurality of opticalmodulators with sloped waveguides and multi-segmented electrodes coupledtogether.

DETAILED DESCRIPTION

Referring now to FIG. 4, there is illustrated an optic modulator 400implemented in accordance with a first embodiment of the presentinvention. As illustrated, the optic modulator 400 comprises a waveguideand electrode structure 403 implemented on a substrate 430, and an RFinput terminal 402 and impedance 404 which are located on a printedcircuit board 401. The substrate 430 may be, e.g., lithium niobatecrystal or a similar optic material that is conductive to light and hasboth electro-optic and piezoelectric properties.

The waveguide and electrode structure 403 comprises an input waveguidesegment 410, a first Y-branch waveguide coupler 412, upper and lowersloping waveguide segments 414,416, a second Y-branch waveguide coupler418, and an output waveguide segment 419.

During operation, an input light wave typically having a power levelexpressed in milliwatts is applied to the input waveguide segment 410,for carrying the light wave to be divided into upper waveguide segment414 and lower waveguide segment 416 by the first Y-branch waveguidecoupler 412. The light wave traveling through the upper waveguidesegment 414 travels through an upper branch of the second Y-branchwaveguide coupler 418 and through the output waveguide segment 419 toprovide a light wave output. Similarly, the light wave traveling throughthe lower waveguide segment 416 travels through a lower branch of thesecond Y-branch waveguide segment 418 and through the output waveguidesegment 419. Note that waveguide segments 410, 412, 414, 418 and 419form a continuous waveguide, as do waveguide segments 410, 412, 416,418, 419.

As illustrated in FIG. 4, a pair of relatively narrow elongated upperouter electrodes 406, 407 are positioned relatively close to the top ofthe upper longitudinal waveguide segment 414 and are shaped to followthe sloping contour of the waveguide segment 414. Similarly, a pair ofnarrow elongated lower outer electrodes 408, 409 are positionedrelatively close to the bottom of the lower longitudinal waveguidesegment 416 and are shaped to follow the sloping contour of thewaveguide segment 416.

A pair of narrow elongated upper and lower center electrodes 420, 421,are positioned relatively close to the bottom of the upper longitudinalwaveguide segment 414 and the top of the lower longitudinal waveguidesegment 416. Electrodes 420, 421 are shaped to follow the slopingcontour of the waveguide segments 414, 416, respectively. An electrodesegment 422 is used to couple the upper and lower center electrodes 420,421 together. As illustrated, the portion of the center electrodes 420,421 which are not coupled together by the electrode segment 422 areseparated from each other by a portion of the substrate material 430.The electrode segment 422 is coupled to the input terminal 402. In thismanner, a modulating or instantaneous voltage V(t) is connected toelectrode segments 422, 420, 421. A DC bias voltage may also beconnected to electrode segments 422, 420, 421. A terminating impedance404, located on the printed circuit board 401, is coupled to the inputterminal 402 and to ground.

The upper and lower outer electrode pairs (406, 407), (408, 409),respectively, are each connected to a source of reference potential,ground in this example. In the illustrated embodiment, electrodesegments 411, 413 connect the upper and lower outer electrode pairstogether to insure that they are at the same reference potential.

The waveguide and electrode elements may be formed on the substrate 430with the waveguide segments being formed by, e.g., diffusing titaniuminto the substrate 430 and the electrodes may be formed by, e.g.,depositing a conductor on the substrate.

The light wave signal applied to the input waveguide segment 410 issubstantially equally divided by the first Y-branch waveguide coupler412. Thus, one half portion travels though the upper waveguide segment414 and another one half portion through the lower waveguide segment416. Light traveling through the upper waveguide segment 414 passesbetween the upper outer electrode pair 406, 407 and the upper innerelectrode segment 420. Light traveling through the lower waveguidesegment 416 passes between the lower outer electrode pair 408, 409 andthe lower inner electrode segment 421. The modulating voltage V(t)causes the index of refraction of the substrate 430 to change. Thisresults in a change in the speed of the light passing through the upperand lower waveguide segments. The change in the speed in the upperwaveguide segment 414 is opposite the change in speed which occurs inthe lower waveguide segment 416. Accordingly, if the speed in the upperwaveguide segment increases in response to a change in V(t), the speedin the lower waveguide segment will decrease in response to the samechange in V(t).

The phase difference created between the light traveling though theupper waveguide segment 414 and the lower waveguide segment 416modulates the light. The light from the upper waveguide segment 414 andthe lower waveguide coupler 416 pass through the second Y-branchwaveguide segment 418 and are combined together to form the modulatedlight signal output by the output waveguide segment 419. The modulatedlight output signal includes only the in-phase components of themodulated light since, in this example, the second Y-branch waveguidecoupler 418 serves as a modulator which absorbs, into the substrate 430,components of the modulated light that are in phase quadrature.

As will be apparent to one of ordinary skill in the art, the opticmodulator 400 may be characterized as a lumped element electrode type ofmodulator which includes sloped waveguides to reduce acoustic effects,e.g., ripple, on the quality of the modulated light. As discussed above,the cutoff frequency of this type of modulator is determined by theequation:

    f.sub.c =1/(RC)

where C is the capacitance of the entire electrode structure and R isthe resistance of the drive source, e.g., the resistance 404.

As is apparent from a comparison of the electrode structures illustratedin FIGS. 3 and 4, the embodiment of the present invention illustrated inFIG. 4 uses what may be characterized as a multiplate center electrodestructure comprising electrode segments 420, 421 and 422 as opposed tothe solid center electrode structure found in the known devices.

By implementing the center electrode structure in such a manner, thecapacitance effect is reduced. This reduces the overall capacitance C ofthe entire electrode structure of the modulator 400. By using amultiplate center electrode assembly as opposed to a solid centerelectrode, a reduction of the overall capacitance C may be achieved withlittle or no adverse impact on the ability to modulate the light wavesignal being passed through the optical modulator 400.

Accordingly, the multiplate center electrode assembly of the presentinvention permits higher frequency signals to be modulated than would bepossible using a similar optical modulator having a solid centerelectrode. Thus, in accordance with one embodiment of the presentinvention, multiplate center electrodes are used to produce modulatorswith larger bandwidths than one possible with similar modulators usingsolid center electrodes.

An optic modulator 500 implemented in accordance with another embodimentof the present invention is illustrated in FIG. 5. Elements that are thesame as or similar to those of FIG. 4 are identified using the samereference numbers as used in FIG. 4 and will not be described again indetail for the sake of brevity.

Referring now to FIG. 5, it can be seen that the optic modulator 500comprises a printed circuit board 501. It also comprises a waveguide andelectrode assembly 503 implemented on a substrate 430. Unlike thepreviously described optic modulators, the optic modulator 500 uses anelectrode structure which comprises a plurality of individual electrodesegments 540, 542, 544. It also uses sloped waveguides to reduceacoustic effects and, in the illustrated embodiment, multiplate centerelectrodes to minimize the effective capacitance. Accordingly, thewaveguide and electrode assembly 503 may be described as an opticmodulator having sloped waveguides and a segmented electrode.

Each electrode segment 540, 542, 544, is defined by an area orcompartment formed by an outer electrode 570. As illustrated in FIG. 5,a series of relatively narrow elongated upper outer electrode segments571, 572, 573, 574, 575, 576 form the upper portion of the outerelectrode 570. The upper portion of the outer electrode 570 ispositioned relatively close to the top of the upper longitudinalwaveguide segment 414 and is shaped to follow the sloping contour of thewaveguide segment 414. A lower portion of the outer electrode 570 isformed by a series of relatively narrow elongated lower outer electrodesegments 584, 585, 586. The lower portion of the outer electrode 570 ispositioned relatively close to the bottom of the lower longitudinalwaveguide segment 416 and is shaped to follow the sloping contour of thewaveguide segment 416.

Front and rear side segments 580, 583 and partition segments 581, 582 incombination with the upper and lower segments of the electrode 570define the electrode segments 540, 542, 544.

The first electrode segment 540 is defined by outer electrode segments571, 572, 581, 584 and 580. The second electrode segment 542 is definedby outer electrode segments 573, 574,582, 585, and 581. The thirdelectrode segment 544 is defined by outer electrode segments 575, 576,583, 586 and 582.

Each electrode segment 540, 542, 544 includes a multiplate centerelectrode assembly. The center electrode assembly of the first electrodesegment 540 comprises a first pair of narrow elongated upper and lowercenter electrodes 550, 551 that are positioned close to the bottom ofthe upper longitudinal waveguide segment 414 and the top of the lowerlongitudinal waveguide segment 416. The electrodes 550, 551 are shapedto follow the sloping contour of the waveguide segments 414, 416,respectively. A coupling electrode segment 552 is used to couple theupper and lower center electrodes 550, 551 to each other and to a firstlead 590.

The center electrode assemblies of the second and third electrodesegments 542, 544 are implemented in a manner that is similar to that ofthe center electrode assembly of the first electrode segment 540. Thecenter electrode assemblies of the second and third electrode segments542, 544 comprise a first pair of narrow elongated upper and lowercenter electrodes (554, 555) (558, 559). The upper center electrodes554, 558 are positioned close to the bottom of the upper longitudinalwaveguide segment 414 while lower center electrodes 555, 559 arepositioned close to the top of the lower longitudinal waveguide segment416, respectively. The electrodes (554, 558), (555, 559) are shaped tofollow the sloping contour of the waveguide segments 414, 416,respectively. A coupling electrode segment 556, 576 is used to couplethe electrodes (554, 555), (558, 559) of the upper and lower centerelectrode pairs to each other and to second and third leads 591, 592,respectively.

As discussed above, the center electrode design of the present inventionreferred to herein as a multi-plate design provides for a lowercapacitance than when a solid center electrode is used. However, itshould be noted that the embodiment of the present invention illustratedin FIG. 5, may be implemented using either solid or multiplate centerelectrodes.

As illustrated, the outer electrode 570 is coupled to a referencepotential, e.g., ground in the embodiment illustrated in FIG. 5.

Each of the first through third leads 590, 591, 592 ms coupled to acorresponding modulating voltage source located on the printed circuitboard 501. As illustrated in FIG. 5, the printed circuit board 501comprises an RF input terminal 502 which is coupled to a first inductor531 having an inductance of L/2 where L is a positive number.Accordingly, the output side of the first inductor serves as the sourceof the voltage used to modulate the center electrode assembly of thefirst electrode segment 540. The inductor 531, in turn, is coupled tothe first lead 590 and to a second inductor 532 having an inductance ofL. The output of the second inductor 532 is coupled to the lead 591 andto a third inductor 533 having an inductance of L. Accordingly, theoutput side of the second inductor 532 serves as the source of voltageused to modulate the center electrode assembly of the second electrodesegment 542.

The third inductor 533 has an output coupled to the third lead 592 andan input of the fourth inductor 544. The fourth inductor 544 has aninductance of L/2 and, in turn, is coupled to a terminating impedance504. The terminating impedance 504 is coupled to ground as is the outerelectrode 570 of the waveguide and modulator structure 503.

As discussed above, the modulator 500 of the present invention utilizessloped waveguides and a segmented electrode configuration to provide anoptical modulator with a large bandwidth and a relatively flat frequencyresponse. The modulator 500 has the advantage that it achieves thishighly desirable result without the need for a complicated buffer layerand/or thick film metallization technology. In addition to having alarge bandwidth and flat frequency response the modulator 500 of thepresent invention also has the advantage of having a high modulationsensitivity.

In the segmented electrode embodiment of FIG. 5, the complete electrodestructure is divided into multiple lumped element segments 540, 542,Electrically, the optic modulator 500 acts as a low pass ladder networkwhere each of the first through third electrode segments 540, 542, 544have interaction lengths l₁, l₂, and l₃, respectively, and a capacitanceof C. In the illustrated embodiment, the electrode segments 540, 542,544 are designed so that l₁, l₂, l₃ are all the same length, l. Thefirst through third electrode segments 540, 542, 543 are connected withan effective inductance of L and terminated into the impedance 505, e.g.a load resistor having a resistance of R. For optimal matching, R shouldbe close to the external impedance, e.g., usually 50 ohms, and thefollowing equation should also be satisfied:

    R=sqrt(L/C)

The principle cut off frequency of the segmented electrode modulator 500will be:

    f.sub.c =1/(RC)

which is the same as for a single element, while the sensitivity isproportional to (n times l) where n is the number of electrode segments540, 542, 544. Accordingly, the sensitivity of the segmented electrodeoptic modulator is increased as compared to a single electrodeembodiment, by a factor equal to the number of electrode segments 540,542, 544, e.g., 3 in the case of the embodiment illustrated in FIG. 5.While 3 electrode segments are used, it is to be understood that anynumber of n segments may be used.

It should be noted that the optical modulation response of the segmentedelectrode modulator 500 does not necessarily follow the matchingresponse given by the equivalent low pass filter response. The opticalresponse is also restricted by the difference in delay of the electronicsignal as compared to the light wave signal between the first electrodesegment 540 and the last electrode segment 544. This characteristic ofthe segmented electrode modulator 500 is similar to that of modulatorsimplemented using a traveling wave configuration.

The critical frequency for a segmented electrode modulator is:

    f.sub.cd = 1/(RC)!× 1/ (n-1)(1-a)!!

where

    a=l.sub.t /(c.sub.1 RC)

and

l_(t) =effective total interactive electrode length

    =l.sub.1 +l.sub.2 +l.sub.3 ;

C₁ =light wave velocity in the waveguide; a is also approximately givenby:

    a˜c.sub.e Z.sub.e /(c.sub.1 Rk)

where:

c_(e) =microwave velocity along the electrode structure;

z_(e) =characteristic impedance of an equivalent coplanar transmissionline; and

k=(n-1)l/l_(t) =electrode "fill factor".

For proper operation of the optic modulator 500, it is hence requiredthat:

    f.sub.cd >=f.sub.c.

By utilizing sloped waveguides in combination with a segmentedelectrode, the optic modulator 500 of the present invention offers theadvantages of a broad, flat frequency response with a minimum ripple dueto acoustic effects.

An optic modulator 600 implemented in accordance with another embodimentof the present invention using sloped waveguides and a segmentedelectrode is illustrated in FIG. 6. Elements of the optic modulator 600which are the same as, or similar to those of FIG. 5, are identifiedusing the same reference numbers and will not be described again indetail.

Referring now to FIG. 6, it can be seen that the optic modulator 600comprises a first printed circuit board 601, a second printed circuitboard 650, and a waveguide and electrode assembly 603. The waveguide andelectrode assembly is 603 implemented on the substrate 430. The firstprinted circuit board 601 is an RF matching circuit board which suppliesmodulating voltage signals to the waveguide and electrode assembly 603.

The first printed circuit board 601 includes an RF input transmissionline 613, a plurality of bond wires 650, 654, 668, 682, a plurality ofhigh impedance transmission line stubs 652, 666, 669, 680, 684, an RFoutput transmission line 606, a chip resistor 607 for matchedtermination of the RF output transmission line 606 and a plurality ofground terminals 661. As illustrated in FIG. 6, the RF inputtransmission line 613 is coupled by the bondwires 650, 654 and the firsthigh impedance transmission line stub 652 to the second high impedancetransmission line stub 666. A bondwire 691 couples the second highimpedance transmission line stub 666 to a center electrode assembly of afirst electrode segment 640 on the substrate 430.

Bondwires 668 and high impedance transmission line stubs 669 couple thesecond high impedance transmission line stub 666 to high impedancetransmission line stub 680. A bondwire 692 couples the transmission linestub 680 to a third center electrode assembly 633. Bondwires 682 andhigh impedance transmission stubs 684 couple transmission stub 680 tothe RF output transmission line 606. The RF output transmission line606, in turn, is coupled to the chip resistor 607 for matchedtermination of the RF output transmission line.

The second printed circuit board 650 is a DC (direct current) biascircuit board which comprises a DC bias transmission line 651. The DCbias transmission line 651 is coupled to a second center electrodeassembly 632 located on the substrate 430.

The waveguide and electrode assembly 603 comprises two RF or modulatingelectrodes segments 640, 644 which are located around a bias electrodesegment 642. The first and second modulating electrode segments 640, 644and the bias electrode segment 642 are formed by compartments created byan outer electrode 670. The outer electrode 670 is coupled to ground 661by a bondwire 660. The first multiplate center electrode assembly 631 islocated inside the first electrode segment 640, the second multiplateelectrode assembly 632 is located inside the bias electrode segment 642and the third multiplate center electrode assembly 633 is located insidethe second modulating electrode segment 644.

The waveguides of the waveguide and electrode assembly 603 are made upof a plurality of waveguide segments including an input waveguidesegment 410, a first Y waveguide coupler 412, an upper longitudinalwaveguide path, a lower longitudinal waveguide path, a second Ywaveguide coupler 418 and an output waveguide segment 419. The upperlongitudinal waveguide path includes a first sloped upper waveguidesegment 620, a generally straight upper waveguide segment 622 and asecond sloped upper waveguide segment 624. Similarly, the lowerlongitudinal waveguide path includes a first sloped lower waveguidesegment 626, a generally straight lower waveguide segment 628 and asecond sloped waveguide segment 630. The first and second upper andlower sloped waveguide segments 620, 624, 626, 630 are symmetricallylocated around the straight upper and lower waveguide segments 622, 628.

Since the bias signal introduced by the bias electrode segment 642 doesnot produce acoustic signals that result in ripple, e.g., the way an RFmodulating signal does, there is no need for the waveguides passingthrough the bias electrode segment 642 to be sloped. Accordingly, in theembodiment illustrated in FIG. 6, only the waveguides and electrodesegments in the first and second RF modulating electrode segments 640,644 are sloped.

The optic modulator illustrated in FIG. 6 has similar advantages interms of bandwidth and flat frequency response to the modulatorillustrated in FIG. 5. However, the optic modulator 600 illustrated inFIG. 6 has the additional advantage of permitting a DC bias level to beadjusted as may be required by applying a bias voltage to the centerelectrode assembly 632 of the DC bias electrode segment 642.

The use of high impedance microwave transmission line stubs in theembodiment of FIG. 6 for external matching rather than discreteinductors provides more reliable high frequency operation as well asease and reproducibility of fabrication than does the use of discreteinductors as illustrated in FIG. 5. The effective inductance may beadjusted by the length and position of the bondwires used to connect thetransmission line stubs.

Referring now to FIG. 7, there is illustrated another optic modulator700 according to an embodiment of the present invention. The opticmodulator 700 is similar to the optic modulator 600. However, unlike theoptic modulator 600, the first printed circuit board 701, of the opticmodulator 700, uses spiral high impedance transmission line stubs 702,703, 704. The use of spiral shaped, high impedance transmission linestubs increases the effective inductance per unit length of the stubs ascompare to straight stubs. This saves space on the printed circuit board701 as compared to the embodiment illustrated in FIG. 6. With theincreased inductance per unit length of the spiral transmission linestubs 702, 703, 704, a large tuning range is more easily accomplished ascompared to when straight transmission line stubs are used.

It should be noted that the bond wires 650, 654, 668, 682 710, 713, and713 used to couple the transmission stubs together can be used foradjusting the matching between the stubs.

As discussed above, linearization of optical modulators is of majorimportance for devices used for, e.g., analog transmission of cabletelevision signals. Many known systems rely on electronic means forlinearization which is readily achievable at bandwidths up to 500 MHz.However, electronic linearization beyond 500 MHz is difficult to obtain.The optical modulators of the present invention described above andillustrated, e.g., in FIGS. 5-7, are capable of bandwidths in excess of500 MHz. Accordingly, there is a need for a linearization method thatcan be used with the modulators of the present invention and applied tosignals of, e.g., 1 GHz and beyond. In order to achieve bestlinearization, a flat frequency response is required. Thus the opticmodulators of the present invention which incorporate sloped waveguidesand segmented electrodes are well suited to achieve good linearizationeven at high frequencies.

In accordance with various embodiments of the present invention, thelinearization approach described in U.S. Pat. No. 5,148,503 and U.S.Pat. No. 5,249,243, which are hereby expressly incorporated byreference, are combined with the optic modulators of the presentinvention which incorporate sloped waveguides and segmented electrodes.The linearization approach described in the above cited patents relieson the cascade coupling of multiple optic modulators in a manner thatcontributes to maximizing the linearity of the signal output by theoptic modulators that are cascaded together.

Referring now to FIG. 8, there is illustrated a cascade coupling ofoptical modulators 810, 820 in a manner designed to increase thelinearity of the signal output by the cascade coupling of the opticmodulators 810, 820. As will be noted from a comparison of FIG. 8, andFIG. 5, the modulators 810, 820 are similar to the optic modulatorpreviously described in regard to FIG. 5. That is, both the first andsecond optic modulators 810, 820 use sloped waveguides and a segmentedelectrode arrangement. As illustrated, both modulators 810 and 820include printed circuit boards 801, 812 having a plurality of first,second, third, and fourth inductors (802, 803, 804, 805) and (814, 816,817) respectively, and a terminating impedance 806, 819, respectively.It should be noted that the inductance and terminating impedance's ofthe first and second modulators 810, 820 may have different values.However, for good linearization over the operating frequency band it isimportant that the two modulators 820, 830 be as identical as possible.

The electrode and waveguide structures of the modulators 810 and 820 areimplemented using three electrode segments (840, 842, 844) (846, 847,848). Because of the similarity of the modulators 810, 820 to that ofthe modulator 500 of FIG. 5, the modulators 810, 820 will not bedescribed in any further detail except in regard to the differencesbetween the modulators 810, 820 and the modulator 500.

As illustrated in FIG. 8, the input waveguide of the modulator 810includes a Y-branch waveguide coupler 412 as does the modulator 500.However, in order to implement a cascade coupling of multiple modulators810, 820 in accordance with the present invention, a Y-branch waveguidesegment is not used at the output of the modulator 810. Instead, ainterferometric coupler structure of the type illustrated in FIG. 8 isused.

The first coupler 850 comprises an upper waveguide segment 838 and alower waveguide segment 836. The upper waveguide segment 838 is coupledto the upper longitudinal waveguide segment 414 of the first modulator810 and the upper longitudinal waveguide segment 414 of the secondmodulator 820 thereby forming a continuous waveguide structure betweenthe upper longitudinal waveguide segments 414 of the first and secondmodulators 802, 810. A lower waveguide segment 836 of the coupler 850 isused to couple the lower longitudinal waveguide segments 416 of thefirst and second modulators 810, 820 together.

During operation, a portion of the light traveling through the lowerhorizontal waveguide segment 836 ideally has half of its light energycoupled into the upper waveguide segment 838. The remaining light fromthe lower waveguide segment 836 then travels into the lower longitudinalwaveguide segment 416 of the second modulator 820. The light passesthrough the upper and lower longitudinal waveguide segments 414, 416 andinto the second coupler 860 which operates in the same manner as thefirst coupler 850. The light output from the upper waveguide segment 839of the second coupler 860 represents the linearized light output of thecascaded modulators 810, 820.

During operation, the amplitude of the modulating voltage applied toeach stage and the level of the DC bias applied to each stage isadjusted for increasing the linearity of the modulated light outputsignal.

The linearization of the light output signal is obtained using a cascadecoupling of the two optic modulators 810, 820 connected to RF signalsRF1 and RF2, respectively. For proper operation the ratio of RF1/RF2should be optimally adjusted, and the first and second waveguidecoupling 850, 860 at the output of the modulators 810, 820 should be ina certain range. The proper ratio for RF1/RF2 and proper output rangesof the modulators 810, 820 are the same as or similar to those discussedin U.S. Pat. No. 5,148,503.

Referring now to FIG. 9, there is illustrated another embodiment of thepresent invention wherein a plurality of optic modulators 910, 920 arecoupled together in a cascade arrangement in order to provide alinearized light output signal. The optic modulators 910, 920 aresimilar in design to the modulator 600 described in regard to FIG. 6.The modulators 910, 920 each include two modulating electrode segments940, 944 and a bias electrode segment 947. Unlike the embodimentillustrated in FIG. 6, in the FIG. 9 embodiment, the bias electrodesegment 947 includes sloped waveguides and sloped electrodes.

The optic modulators 910, 920 are coupled together in a cascadearrangement in the same manner as the optic modulators 810, 820illustrated in FIG. 8.

In the FIG. 9 embodiment, an RF signal is supplied to an RF input 912which is coupled to an RF power splitter 915. The RF power splitter 915,in turn, is coupled to the RF input of the first modulator 910 and to aninput of an adjustable attenuator having, e.g., a resolution of 0.1 dBor better. During use, the adjustable attenuator is adjusted for optimallinearization. An output of the adjustable attenuator 916 is coupled tothe RF input of the second modulator 920. With regard to the DC biassignal, a bias adjustment circuit 928 is used to supply a bias signal tothe bias transmission lines 651 of the first and second modulators 910,920.

Using the cascade coupling arrangement illustrated in FIG. 9,linearization may be achieved at frequencies well above 1 Ghz.

As discussed previously, the use of high impedance microwavetransmission line stubs for external matching rather than discreteinductors offers superior reliability and reproducibility than does theuse of discrete inductors. The effective inductance may be adjusted bythe length and positioning of the bondwires used to connect thetransmission line stubs.

Referring now to FIG. 10, there is illustrated yet another embodiment ofthe present invention wherein a plurality of optic modulators 1010, 1020are coupled together in a cascade arrangement in order to provide alinearized light output signal. The optic modulators 1010, 1020 aresimilar to the modulators 910 and 920 but use spiral transmission linesto reduce the amount of space required to implement the modulatingcircuit boards. Accordingly, the details of the modulators 1010, 1020will not be discussed in any further detail except to note that elementsin FIGS. 9 and 10 which bear the same reference numerals as element ofother figures are intended to refer to the same or similar elements.

While the above described embodiment of the present invention arefrequently described using examples of segmented electrodes having threesegments, it is to be noted that the teaching of the present inventionmay be applied to modulators having any number of segments. Furthermore,any number of modulators may be coupled together in accordance with theabove described approach to provide a linearized light output from thecascaded modulators.

What is claimed is:
 1. An optic modulator, comprising:a substrate of anelectro-optic material; a pair of first and second waveguides formed insaid substrate, extending substantially coextensively with one anotherin the longitudinal direction and separated by a transverse distancethat gradually varies along the course of the first and secondwaveguides; a first elongated outer electrode segment located in closeproximity to the first waveguide and extending in the longitudinaldirection of the first waveguide; a second elongated outer electrodesegment located in close proximity to the second waveguide and extendingin the longitudinal direction of the second waveguide; a first elongatedinner electrode segment located in close proximity to the firstwaveguide and extending in the longitudinal direction of the firstwaveguide; a second elongated inner electrode segment located in closeproximity to the second waveguide, the first and second elongated innerelectrode segments being positioned between the first and secondwaveguides and being separated along at least a portion of their length,the first waveguide being positioned between a first electrode pairformed by the first outer and inner elongated electrode segments and thesecond waveguide being positioned between a second electrode pair formedby the second outer and inner elongated electrode segments; an inputcoupler coupled to the first and second waveguides for introducing twoportions of light, one portion of light being introduced into eachwaveguide; and an output coupler for combining the portions of lightwith one another after the emergence thereof from the first and secondwaveguides to produce a modulated light output signal.
 2. The opticmodulator of claim 1, further comprising:an inner connecting electrodesegment extending in a direction transverse to the longitudinaldirection of the first waveguide for coupling the first and second innerelectrode segments to each other and to a modulating voltage.
 3. Theoptic modulator of claim 2, further comprising first and second outerconnecting electrode segments coupled to both the first and second outerelectrode segments and to a reference potential, the first and secondouter electrode segments and the first and second outer connectingelectrode segments substantially surrounding the inner electrodesegments.
 4. The modulator of claim 3, wherein the first and secondinner electrode segments and inner connecting electrode segment areformed by depositing an electrode material on the substrate and whereinno electrode material is deposited on a substantial portion of thesubstrate surface area located between the first and second innerelectrode segments.
 5. The modulator of claim 4, wherein the spacingbetween the first and second waveguides is at a minimum at both ends ofsaid first and second waveguides.
 6. The modulator of claim 5, whereinthe distance between the first and second waveguides is at a maximumnear the center of the first and second waveguides.
 7. The modulator ofclaim 4, wherein at least 25% of the substrate surface area locatedbetween the first and second inner electrodes is free of electrodematerial.
 8. The modulator of claim 7, wherein the substrate compriseslithium niobate.
 9. The modulator of claim 8, wherein the first andsecond outer electrode segments and the first and second outerconnecting electrode segments form a ground plane.
 10. The modulator ofclaim 9, wherein said first and second outer electrode segments arecontoured to follow the shape of the first and second waveguides,respectively.
 11. The modulator of claim 10, wherein the first andsecond inner electrode segments are contoured to follow the shape of thefirst and second waveguides, respectively.